1. Field of the Invention
This invention relates to a polycarbodiimide resin-containing adhesive and also to a flexible printed circuit board fabricated using the polycarbodiimide resin-containing adhesive and a process for the fabrication of the printed circuit board.
2. Description of the Related Art
Low molecular carbodiimides such as dicyclohexylcarbodiimide, diisopropylcarbodiimide and di-p-toluoylcarbodiimide are highly reactive reagents generally used as a reagent for dehydration and condensation. These low molecular carbodiimides are used in the synthesis of esters, peptides or the like.
A polycarbodiimide having plural carbodiimide bonds in its molecule is generally prepared by the decarboxylation condensation of a diisocyanate in a proper solvent in the presence of a carbodiimidation reagent.
It is disclosed in J. Org. Chem., 28, 2069(1963) and U.S. Pat. No. 2,941,966 that the polycarbodiimide obtained from an aromatic diisocyanate is particularly useful as a heat-resistant macromolecule.
Flexible printed circuit boards (FPC) have so far been fabricated using an adhesive such as an epoxy resin (Japanese Patent Provisional Publication Nos. 59-89380, 60-260669, 6079079) or urethane resin between a copper foil and an aromatic polyimide film which has inherently inferior adhesiveness.
The flexible printed circuit boards fabricated using an adhesive such as epoxy resin or urethane resin are however accompanied with the problems that when such flexible printed circuit boards are exposed to high temperatures in the subsequent soldering step, there occurs blisters or peeling of the adhesive layer, or short cut occurs at the interconnection due to the plastic deformation of the adhesive layer.
With a view to overcoming such problems, polyimide base or polybenzimidazole base adhesives having excellent heat resistance have been developed. These polyimide or polybenzimidazole base adhesives are not always suited for the fabrication of a flexible printed circuit board, because polyimide or polybenzimidazole resins used for them are expensive, and a special high-boiling-point solvent contained in such adhesives requires severe adhesion conditions at high temperature and high pressure.
There is a report (Japanese Patent Provisional Publication No. 4-36369) on the preparation of a heat-resistant adhesive from a polycarbodiimide resin. That adhesive is aimed at the adhesion of not an organic material but an inorganic material. Furthermore, the report only includes the finding of adhesion performances at temperatures higher than 350.degree. C. and it does not suggest that the adhesive is suitable for the fabrication of a flexible printed circuit board.
In addition to the above-described adhesives, various adhesives such as those made of a fluorine resin, polyamide-imide resin, silicone resin, epoxy-novolak resin, epoxyacrylic resin, nitrile-rubber phenol resin or polyester resin have been developed. None of them, however, satisfy both heat resistance and high-temperature adhesive strength.
In addition, in recent years, fine patterning and multi-layerization of interconnection circuit in a flexible printed circuit board are under way, as electronic devices become smaller, lighter and high-performanced. Accordingly, it is desired to make the adhesive flame-retardant without using a halogen compound which has adverse effects on the environment. Also, there is a demand for an improvement in the heat resistance and adhesion properties of an adhesive to be employed so as to avoid blisters and peeling, even if the final product is subjected to various high-temperature treatment steps, and to prevent curling or plastic deformation of an adhesive layer from occurring. Curling and deformation are known to cause such problems as short circuiting of interconnections.